Novel thick laminate fabrication method

ABSTRACT

The invention is a processing technique which allows Fiber Reinforced Polymer laminates of any arbitrary thickness to be manufactured. Thick laminates processed in this fashion will not exhibit adverse process induced residual stresses which serve to reduce or limit laminate operability, performance and service life. This invention provides a means by which conventional processing problems associated with very thick laminate construction can be avoided, thereby ensuring a high degree of thick laminate part quality and performance

RELATED APPLICATIONS

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FEDERALLY SPONSORED RESEARCH

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SEQUENCE LISTING

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BACKGROUND OF THE INVENTION

The invention relates to producing thick composite laminates, and specifically to addressing process problems associated with thick laminate manufacture.

Fiber reinforced polymer (FRP) matrix construction has been widely used in the aerospace industry for quite some time because it exhibits a much higher specific strength (i.e. strength divided by density) compared to isotropic metals. The weight savings associated with FRP construction offers significant structural performance advantages as well as ballistic protection advantages in defense as well as commercial applications.

As structural performance and ballistic threat requirements continue to increase, the need for thicker and thicker FRP construction mounts. Where aerospace applications have successfully used thin FRP laminates, other applications in civil engineering, transportation and ballistic protection have pushed the FRP processing envelope toward thicker and thicker laminates.

Thick laminate construction exceeding two inches in thickness has typically been difficult to accomplish in a repeatable, high quality fashion. Fabrication problems associated with the use of thermoset resin systems such as epoxy, polyester and vinyl ester resins are prevalent in the fabrication of thick laminates. This is primarily due to the high exothermic reaction of the resin matrix as polymerization occurs during curing of laminates using reinforcing fibers exhibiting low thermal conductivity. Laminates fabricated using low thermal conductivity fibers such as E-Glass and S-2 Glass prevent an exothermic reaction from properly dissipating the heat generated from the exotherm.

Consequently, in the case of thick glass fiber reinforced laminate construction, the high fiber volume fraction impedes adequate heat transfer during evolution of the exothermic reaction created during polymerization of the resin. The internal heat generated by the exothermic reaction of the resin develops thermal stresses in the FRP laminate during the curing process. These thermal stresses are of sufficient magnitude to cause the laminate to crack before the laminate becomes fully cured. The thicker the laminate the higher the thermal gradients that are developed during the curing process. It is these high tensile process induced residual stresses which adversely affect laminate quality and performance.

Thick panels of composite materials are becoming vital for defense as well as commercial applications, particularly as such panels are well suited for explosive resistant applications that are extremely relevant in an age of global terrorism. Thus there is a critical need to fabricate very thick, as thick as 20 inches or more, glass fiber reinforced laminates without the formation of interlaminar flaws, air voids or excessive tensile residual stresses, all of which impair design functionality, service life, performance and fitness for purpose. It is the object of this invention to provide a means by which all the aforementioned problems associated with very thick laminate construction can be avoided, thereby ensuring a high degree of laminate quality and performance.

BRIEF SUMMARY OF THE INVENTION

The invention is a method of producing a thick laminate assembly of Fiber Reinforced Polymer (FRP) panels. The method includes placing an electric heating element on a surface of a pre-cured first panel such that the heating element can be controlled external to the laminate assembly, covering the surface of the first panel and electric heating element with thermoset resin and placing a second pre-cured panel on top of the first panel. The electric heating element is used as the heat source to cure the thermoset resin, thereby bonding the two panels together into a laminate assembly.

In one version, the method includes bonding additional panels to the laminate assembly until an assembly of desired total thickness is achieved, using an electric heating element as the heat source for curing the thermoset resin used to bond additional panels to the assembly. The panels may be added sequentially or the entire assembly can be set-up and cured simultaneously.

In one embodiment, the electrical resistance element is at least one heating wire laid over a surface area of the panel in a serpentine pattern, such that both ends of the wire can be accessed external to the panels. In one version, the turns of the pattern are within the surface area of the panel. In another version, the turns are around pins external to the surface area of the panel.

In one aspect, the method includes a step of sealing the joint line between panels when the resin is heated to a point of minimum viscosity. In a further aspect, the assembly is vacuum bagged during the curing process.

In one aspect, the method includes at least one thermocouple placed between the panels to monitor temperature during the cure cycle. In one version, two thermocouples are installed at opposite diagonal corners of the panel.

In another embodiment, the invention is a method of producing a laminate assembly of Fiber Reinforced Polymer (FRP) panels including placing an electric heating element on a surface of a first panel such that the heating element can be controlled external to the laminate assembly, placing a film adhesive with the heating element, and placing a second panel on top of the first panel. The electric heating element is used as the heat source to heat the film adhesive, thereby bonding the two panels together into a laminate assembly. In one version, the film adhesive is a single layer woven fabric, pre-impregnated with thermoset resin and partially cured. The film adhesive implementation supports the various embodiments, aspects, and versions of the above implementation employing the resin.

The invention also includes an adhesive film for joining panels together including; a single layer woven fabric, pre-impregnated with thermoset resin and partially cured, and at least one heating wire. The wire is woven in a serpentine pattern into the fabric which constitutes the film adhesive layer, such that the wire can be accessed on both ends external to the fabric, and the wire, resin and fabric are packaged as one unit which can be placed between panels to be joined. In one version the film adhesive layer includes at least one thermocouple mounted on the fabric with connections external to the film.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the invention will be better understood by referring to the accompanying drawings.

FIG. 1 illustrates the heating element on a single pre-cured panel.

FIG. 2 shows two panels bonded together according to the invention.

FIG. 3 illustrates the heating wire woven into a sheet of fabric for a film adhesive embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The invention eliminates the formation of high tensile process induced residual stresses by bonding pre-cured FRP laminate subassemblies, where each pre-cured laminate panel subassembly does not exceeding a specified maximum thickness, typically not more than two inches. The individual FRP panel subassemblies may be manufactured using any number of different processing methods (e.g. resin transfer molding, compression molding, resin vacuum infusion, wet hand lay-up, etc.) as long as surface irregularities and unevenness are controllable. Consequently, tooling must be adequately designed to minimize part distortion after the pre-cured fabricated subassembly is removed from its tool.

The pre-cured laminate subassemblies are then bonded to one another until the desired total laminate thickness is achieved, either incrementally, i.e. by bonding one subassembly to another one at a time, or collectively by stacking several panel subassemblies together to the required thickness. Conventional approaches such as autoclaves, forced air, convection ovens or heating blankets heat the entire part thereby causing high thermal gradients when using low thermal conductivity reinforcing fibers. The key to the invention is the method by which the thermoset resin, located between adjacent panel subassembly surfaces, is cured without developing adverse process induced residual stresses during curing of the bond line between panels.

Referring to FIG. 1, the invention involves the use of an electric resistance heating element, preferably a heating wire 2 which in one embodiment is laid in a serpentine fashion over the surface area of one of the panel subassemblies 1 to be bonded as shown.

Prior to spreading of the resin onto the first pre-cured FRP panel subassembly 1, the heating wire 2 may be laid down directly onto the surface of the first subassembly to be bonded. Referring to FIG. 2, after the resin is metered onto the wired surface of the first subassembly 1, the second pre-cured FRP panel subassembly 3 is lowered onto the first subassembly 1. The joint line between panels is preferably sealed to avoid resin leakage when the heated resin reaches minimum viscosity. A vacuum bag may be placed over both subassemblies to achieve proper part consolidation. An electric current is applied to the heating wire 2. Resin temperature may then be monitored throughout the resin cure cycle via thermocouples, preferably two thermocouples installed at opposite diagonal corners of the panel interface.

The turns of the heating wire may occur on the surface of the first pre-cured FRP subassembly or may be achieved by turning the heating wire around pins or dowels located outside of the part, preferably along two parallel edges some short distance away from the panel subassembly.

In another embodiment, film adhesive is used to bond adjacent pre-cured subassemblies together rather than neat resin. Film adhesive is a single layer of woven fabric which has been pre-impregnated with a prescribed amount of resin and partially cured (i.e. B-staged). Film adhesive must be kept refrigerated in order to avoid curing of the resin when exposed to ambient temperature conditions. The thickness of the film adhesive is selected based on surface irregularities and the relative unevenness of mating surfaces. The film adhesive is cured in the same fashion as the neat resin via resistance heating from the electric heating wire placed between one of the pre-cured panel subassemblies and the film adhesive. The film adhesive and the resistance heating wire are sandwiched between two pre-cured panel subassemblies. The perimeter of the interface between adjacent subassemblies may be sealed to avoid resin leakage when minimum viscosity is reached. A vacuum bag may be placed over both subassemblies to achieve proper part consolidation. An electric current is applied to the heating wire. Resin temperature may be monitored throughout the resin cure cycle via thermocouples, preferably two thermocouples installed at opposite diagonal corners of the panel interface.

In an alternative implementation shown in FIG. 3, the electric resistance heating wire 2 is woven directly into the fabric of the film adhesive 4. The film adhesive with integrated heating wire is then placed between pre-cured FRP panel assemblies and cured in the same fashion as described above. 

1. A method of producing a thick laminate assembly of Fiber Reinforced Polymer (FRP) panels comprising; placing an electric heating element on a surface of a first panel such that the heating element can be controlled external to the laminate assembly, covering the surface of the first panel and electric heating element with thermoset resin; and placing a second panel on top of the first panel, wherein the electric heating element is used as the heat source to cure the thermoset resin, thereby bonding the two panels together into a laminate assembly.
 2. The method of claim 1 further comprising the step of bonding additional panels, sequentially or simultaneously, to the laminate assembly until an assembly of desired total thickness is achieved, using electric heating elements as the heat sources for curing the thermoset resin used to bond additional panels to the assembly.
 3. The method of claim 1 wherein the electrical resistance element is at least one heating wire laid over a surface area of the panel in a serpentine pattern, such that both ends of the wire can be accessed external to the panels.
 4. The method of claim 3 wherein the turns of the pattern are within the surface area of the panel.
 5. The method of claim 3 wherein the turns are around pins external to the surface area of the panel.
 6. The method of claim 1 further comprising a step of sealing the joint line between panel when the resin is heated to a point of minimum viscosity.
 7. The method of claim 1 wherein the assembly is vacuum bagged during the curing process.
 8. The method of claim 1 wherein at least one thermocouple is also placed between the panels to monitor temperature during the cure cycle.
 9. The method of claim 8 wherein two thermocouples are installed at opposite diagonal corners of the panel.
 10. A method of producing a thick laminate assembly of Fiber Reinforced Polymer (FRP) panels comprising; placing an electric heating element on a surface of a first panel such that the heating element can be controlled external to the laminate assembly, placing a film adhesive with the heating element; and placing a second panel on top of the first panel, wherein the electric heating element is used as the heat source to heat the film adhesive, thereby bonding the two panels together into a laminate assembly.
 11. The method of claim 10 wherein the film adhesive is a single layer woven fabric, pre-impregnated with thermoset resin and partially cured.
 12. The method of claim 10 further comprising the step of bonding additional panels to the laminate assembly, either sequentially or simultaneously, until an assembly of the desired total thickness is achieved, using electric heating elements as the heat sources for heating the film adhesive used to bond additional panels to the assembly.
 13. The method of claim 10 wherein the electrical resistance element is at least one heating wire, such that the wire is laid over a surface area of the panel in a serpentine pattern and both ends of the wire can be accessed external to the panels.
 14. The method of claim 13 wherein the turns of the heating wire pattern are within the surface area of the panel.
 15. The method of claim 13 wherein the turns of the heating wire are around pins external to the surface area of the panel.
 16. The method of claim 10 further comprising a step of sealing the joint line between panel when the film is heated to a point of minimum viscosity.
 17. The method of claim 10 wherein the assembly is vacuum bagged during the heating process.
 18. The method of claim 10 wherein at least one thermocouple is also placed between the panels to monitor temperature during the heating cycle.
 19. The method of claim 10 wherein two thermocouples are installed at opposite diagonal corners of the panel.
 20. An adhesive film for joining panels together comprising; a single layer woven fabric, pre-impregnated with thermoset resin and partially cured; and, at least one heating wire, wherein the wire is woven into the fabric in a serpentine pattern, such that the wire can be accessed on both ends external to the fabric, and the wire resin and fabric are packaged as one unit which can be placed between panels to be joined.
 21. The adhesive film of claim 20 further comprising at least one thermocouple mounted on the fabric with connections external to the film. 